After nearly 100 years of rumors and historical speculation, DNA testing has confirmed that President Warren Harding had a child out of wedlock – his only biological child – with mistress Nan Britton.

Britton first came forward publically with the claim that her daughter, Elizabeth Ann, was Harding’s daughter in a 1927 autobiography “The President’s Daughter.” In her account, Britton detailed a steamy six-year-long affair with the 29th president, including one encounter in a White House closet, before his untimely death in 1923.

At the time of its publishing, the book was met with public ridicule and widely discounted as the stuff of pornographic fiction. Britton was labeled a “sex pervert” and “degenerate,” and a book was even written to counter her claims about Harding, who was married to Florence Mabel Harding.

Harding historian James Robenalt compares Britton to the Monica Lewinsky of her time.

“Nan Britton was someone who had to live through a lot of attacks … and I think her story was a lot like Monica Lewinsky because there was a real shaming process,” Robenalt said. “She was just picking up for her daughter, who we now know was Harding’s daughter, and she was just viciously attacked for it.”

Getty Images/ Courtesy Neil K Evans and Hahn Loeser LLP

Warren Harding and his out-of-wedlock child, who was just proven to be his daughter through DNA testing.

But recent DNA testing by Ancestry has proven that James Blaesing, the son of Harding’s biological daughter, Elizabeth Ann, is the second cousin of the president’s grandnephew Peter Harding and his grandniece Abigail Harding.

The search to prove Britton’s story has been a six-year journey, prompted by Harding's grandnephew Peter, who said he resolved to solve the mystery after he discovered a copy of "The President's Daughter" among his sister's possessions that had previously belonged to their father, who was Harding's brother.

Harding read the book back-to-back with Robenalt's 2009 book, “The Harding Affair: Love and Espionage during the Great War,” that detailed a long-running affair between Harding and another mistress Carrie Phillips and included a trove of love letters the president wrote to Phillips.

"I read [the books] back to back and I realized they were talking about the same man," Peter Harding told ABC News today.

All his life, Harding said, his family had maintained that Britton was a "delusional woman who believed in a fantasy." The family believed that President Harding, who had mumps as a child, was sterilized by the illness and could not have children.

But upon reading the book, and comparing the love letters Britton described in her book to those written to Phillips and included in Robenalt's book, Harding realized that he "was dealing with a formidable author who was telling the truth."

Unlike Phillips, Britton had no documented proof of the letters Harding had written her, having destroyed the love letters he wrote her at his request.

So Peter Harding reached out to Britton's descendants and found a willing partner in Jim Blaesing, Britton's grandson and now the proven grandson of the 29th president. After a long process of DNA testing, it wasn’t until recently that Ancestry developed biological testing that was precise enough to accurately connect Harding’s grandnephew to Elizabeth Britton’s grandson.

Harding said the decision to undergo DNA testing, and the subsequent proof of familial connection, has been an emotional journey for both families.

"Each family has their own set of beliefs and we had never discussed this in my family and in Jim’s family," he said. "We both broke with our family ideology, and I feared losing friendships within the family. I was violating a lot of family rules to bring this about.”

But now that the familial connection has been proven, Harding said, his family has embraced the news and are now in the process of planning a family reunion with their newly discovered family members.

The importance of the discovery, Robenalt believes, is twofold.

“For one, this family can be united,” Robenalt said. “There are literally grandsons of Harding who don’t know their cousins.”

And, secondly, he hopes the discovery will begin to clear the air around Harding’s scandalized reputation in history.

“This book really ruined Warren Harding’s reputation, and as a result the important lessons of his presidency have been lost,” Robenalt said, who points out that Harding argued for non-interventionist policies before World War I that continue to be relevant following the lessons of the war in Iraq.

“It’s not our job to go in and change other forms of government into a democracy, and he was right,” Robenalt said.

“He’s seen as scandal-ridden and someone without any substance,” he added, “and this story will hopefully ‘put to bed’ all this stuff and the focus on who he was a president.”

Harding said he also believes he has proven something about his grand-uncle's character. Though Britton was 30 years Harding's junior, Peter Harding says the president's extramarital relationships were not the result of womanizing, but love.

"He was not a womanizer," Harding contends. "He had two women in his life, one after the other, he was in love with both of them. And Nan Britton loved him her entire life.”